Used as an exclamation of surprise, or emphasis when describing something out of the ordinary, often situated at the start of a sentence.
Could be used in a similar situation to - oh my god!, wowzers!, or you wouldn't believe this but....

Most commonly used in the east of Scotland, esp Fife.

Sha hoor.. would ye look at the state o that old dear, she's aff her puss on the sherry like!

Sha hoor neebur, I was gan doon the merkat the ither day an a load o young ains peltit me wi yon water balloons, wee buggers totally drenched us like! Ya should o seen me, ah was pure dreepin aw ower the shop!